1. Field of the Invention
The present invention relates to track-number computing equipment for CLV (Constant Linear Velocity) disc drives. This equipment computes the number of tracks between a reference track and a target track in which a desired piece of music or data is recorded when a random access operation is carried out.
2. Prior Art
Digital discs, such as CDs (Compact Discs), use CLV in order to reduce the size of the discs. CLV is an acronym for "Constant Linear Velocity", in which the rotating rate of a disc varies according to the radial position of the pick-up so that a constant linear velocity is achieved. In CLV, a random access operation, such as searching the top of a desired piece of music, is performed by computing the radial displacement of a pick-up from a current position to a target position by using a microprocessor.
FIG. 1 is a diagrammatical view of the data format in which data are recorded on a CD.
As is well known, the data stream is split up into and recorded in frames. A frame consists of frame-synchronization pattern 1, subcode 2, data words 3, and parity words 4. Frame-synchronization pattern 1 is used to identify the top position of each frame, data words 3 are two channel data L and R (left and right) formed by sampling original signals at a sampling rate of 44.1 kHz, and parity words are used for detecting and correcting errors.
Subcode 2, having 8 bits for each frame, consists of 98 eight-bit symbols from 98 frames which constitute 1 block. Each bit of the eight-bit subcoding symbol of one frame corresponds to 8 channels P to W, and so each channel P to W consists of 98 bits. Channel P indicates the beginning of a piece of music by maintaining at least two seconds of "1" level at the beginning, and maintaining "0" level afterwards. Channel Q has three modes: in Mode 1, channel Q records time data, and other data; in Mode 2 and 3, channel Q records a catalogue number of the disc. Channels R to W are not utilized at present.
FIG. 2 is a diagrammatical view of the channel Q in Mode 1. Numeral 10 in FIG. 2 designates piece-number data of 8 bits for indicating the number of the piece. Numeral 11 designates index data of 8 bits, numeral 15 designates time data of 24 bits for indicating the elapsed time from the beginning of the piece. Time data 15 consists of 3 types of eight-bit data: minute data 12 indicating minutes, second data 13 indicating seconds, and frame-number data 14 indicating the number of frames (one second contains 75 frames). Next to time data 15, there are 8 consecutive "0" bits, and absolute time data 19 indicating the total elapsed time from the top of the musical area of the disc. Absolute time data 19 is composed in a manner similar to time data 15, that is, time data 19 consists of minute data 16, second data 17, and frame-number data 18 that indicate the total elapsed time. Thus, the data of channel Q consist of 72 bits, and each piece of data therein is stored in the form of 2 digit BCD (Binary Coded Decimal).
The disc has a lead-in area inside the musical tracks, i.e., at the innermost part of the disc. FIG. 3 shows the format of a table of contents (TOC) provided in the lead-in area. The table of contents records the duration and start time of each piece recorded on the disc as in channel Q described above.
The conventional CD player performs a random access operation as follows by using the data described above:
When the number of a target piece of music is entered by manipulating the operation button, this number is converted into the target position data and is stored into a memory in the CD player. The C player recognizes the current position of the pick-up by reading absolute time data 19 of channel Q. This is achieved by rotating the disc for a certain distance. Then, the CD player compares the current position and the target position to compute the radial distance therebetween by means of a microprocessor, and moves the pick-up to the target position. When the pick-up seems to reach the target position, the CD player reads absolute time data 19 of channel Q again, compares the read data with the target data, and moves the pick-up by a computed distance, thus positioning the pick-up at the target position. The CD player, as described above, repeats the operations of reading absolute time data 19 of channel Q, computing the difference between the current position and the target position, and moving and positioning the pick-up on the target position.
Thus, the positioning of the pick-up is achieved. However, since the data of channel Q are split up into 98 different frames, the CD player must read at least 98 frames to obtain the channel Q data, which takes a considerable length of time. To shorten the time required to reach the target position, the following method is adopted:
First, absolute time data 29 indicating the starting time of a target piece is read from the table of contents shown in FIG. 3. The CD player, using the read time data, computes the distance between the target position and the starting track (i.e., innermost track) of the musical area by using the microprocessor, and moves the pick-up by the distance obtained. In this case, fine adjustment of the positioning of the pick-up is required at the final stage of the positioning, because of the very narrow track pitch of about 1.6 .mu.m.
In this method, the target position of the pick-up is computed as a radial distance from the starting track of the musical area by using the absolute time data. However, the distance cannot be rapidly calculated to a high degree of accuracy in a short time, because the microprocessor is not powerful enough t achieve the high speed computation. As a result, a considerable length of time is required for the pick-up to reach the target position.